Sodium hypochlorite is the most commonly used fungicide for controlling biofouling in aqueous systems including industrial water systems such as water-cooling systems and papermaking processes, but in those cases where superior sterilization performance is required, if the sodium hypochlorite is used in a large amount, then corrosion of the piping and problems of odor may sometimes arise. Accordingly, in these cases, sodium hypobromite is typically used due to its higher level of sterilization performance, but because sodium hypobromite is unstable, techniques in which a bromine compound such as sodium bromide and sodium hypochlorite are mixed together immediately prior to use to generate the sodium hypobromite within the system are typically employed industrially. However, even in these cases, problems including the difficulty of ensuring uniform mixing of the two liquids and corrosion still remain, and therefore a one-liquid stabilized hypobromous acid composition having excellent storage stability is required.
Various oxidizing bromine formulations comprising a bromine stabilizer such as sulfamic acid, bromine, and a hydroxide or the like have been proposed as one-liquid stabilized hypobromous acid compositions.
A technique for activating bromide ions to generate hypobromites by adding a bromine stabilizer and then adding an oxidizing agent such as a hypochlorite has also been proposed. A method using sodium hypochlorite as an oxidizing agent, and then utilizing the hypobromous acid produced upon reaction with a bromine compound is disclosed in Patent Document 1 and Patent Document 2. Patent Document 1 discloses a method of adding sulfamic acid to a premixed solution composed of sodium hypochlorite and a bromine compound, and Patent Document 2 discloses a method of adding a bromine compound to a premixed solution composed of sodium hypochlorite and sulfamic acid. In either case, as the products from the sulfamic acid may decompose, the addition is preferably performed at a temperature of about 10 to 45° C., and is more preferably performed at about 20° C. However, in these methods, chlorine and chloride ions derived from the sodium hypochlorite are retained in the product, meaning problems such as corrosion remain. Further, the hypobromous acid is unstable, and generation of bromic acid as a by-product is problematic.
A method in which reaction is performed using bromic acid as the oxidizing agent instead of a chlorine-based oxidizing agent is disclosed in Example 1 and Example 2 of Patent Document 3. The two equations shown below are disclosed as the reaction mechanism, with the contribution of the bromic acid to the reaction being an essential factor. However, in terms of safety and the like, the use of bromic acid as a raw material in an industrial setting is problematic.2Br−+BrO3−+3H+→3HBrO  (2)HBrO+−O—SO2—NH2→−O—SO2—NH—Br, −O—SO2—NBr2 and other stable oxidizing bromine compounds  (3)
A method of reacting only bromine to obtain an oxidizing bromine-based composition without using an oxidizing agent is disclosed in another embodiment of Example 2 and in Example 3 of Patent Document 3, and in Example 4 of Patent Document 4. However in the other embodiment of Example 2 of Patent Document 3 and Example 4 of Patent Document 4, the corrosiveness is high and a significant problem, and the other embodiment of Example 2 in Patent Document 3 also suffers from problems of a strong irritating odor and poor storage stability. Further, in Example 3 of Patent Document 3, the formation of large amounts of crystals was confirmed, and in terms of generating a large amount of crystals, this method also utilizes the oxidizing power of the bromic acid generated during the reaction, so that the product actually produced by this reaction suffers from a problem of containing significant residual bromic acid.
Moreover, known slime inhibition methods for separation membranes such as RO membranes include methods which use various slime inhibitors. Oxidizing agents such as hypochlorous acid and hypobromous acid are typically representative slime inhibitors, but there is a problem with these inhibitors in that they degrade the membrane.
Although there is a document (Patent Document 5) that discloses the use of a hypobromous acid solution as a temporary reverse osmosis membrane cleaning agent, because the method uses hypobromous acid itself, even temporary use raises the possibility of membrane degradation. Further, this cleaning application is limited to temporary use, and is fundamentally different from a permanent slime-inhibiting treatment that is continuously kept in contact with the reverse osmosis membrane.
One document (Patent Document 6) also describes the injection of hypobromous acid at a stage prior to a reverse osmosis membrane, but this method also simply uses hypobromous acid itself. Further, the method of Patent Document 6 relates to a method for “pretreatment” of the water flowing into the reverse osmosis membrane, and the hypobromous acid within the water is subjected to a reductive decomposition treatment immediately prior to flowing into the reverse osmosis membrane, meaning the treatment is fundamentally different from a slime inhibition treatment that is continuously kept in contact with the reverse osmosis membrane.
On the other hand, a slime inhibition treatment using a combined chlorine-based oxidizing agent such as chlorosulfamic acid or the like prepared by stabilizing hypochlorous acid with sulfamic acid has also been proposed (Patent Document 7). These combined chlorine-based oxidizing agents have minimal degradation effect on the membrane, but the slime inhibitory effect is unsatisfactory.
Further, in RO devices, in order to inhibit scale, the devices are typically operated with the pH adjusted toward the acidic side (for example, a pH of about 4.0) (for example, see Patent Document 8).